The present invention relates to a process and an apparatus for obtaining beams of particles having a spatially modulated density. It more particularly applies to ionic etching and in particular fine and localized etching, in the field of microelectronics, as well as to ion implantation and more specifically localized doping with the aid of such an implantation.
The term "particles" is understood to mean atoms, groups of atoms, molecules, negative ions and positive ions.
The term "beam of particles having a spatially modulated density", is understood to mean a beam of particles for which the density of the particles, in a given cross-section of said beam, is dependent on the considered point of said section. It is possible to have an "all or nothing" spatial modulation or a discontinuous spatial modulation, i.e. a spatial modulation for which any cross-section of the beam is formed from regions containing particles and which are separated from one another. It is also possible to have a continuous spatial modulation, i.e. a spatial modulation for which the density of the particles, in any cross-section of the beam, varies in a continuous manner between the individual points of said section.
A method is known for obtaining a beam of particles having a spatially modulated density, which consists of passing the beam through a reticular or latticed mask, the beam then being focused, when required by the envisaged application. The latticed mask can be constituted by a plate perforated by openings of an adequate size, then giving a discontinuous modulation, or can be constituted by a plate whose absorption for the incident beam varies as a function of the considered point on the plate, then giving an analog spatial modulation.
This method has the disadvantage of making it impossible to realise certain modulations. In addition, the latticed mask used in this method is rapidly deteriorated by the incident particle beam.
Another method is known for obtaining a beam of particles having a spatially modulated density, which consists of rapidly scanning a given portion of the space by means of a fine beam, whilst modulating the intensity of the beam as a function of time, so as to obtain the desired spatial modulation in the space portion scanned by the beam. This other method has the disadvantage of taking a long time and of requiring the formation of a perfectly controlled, fine beam.